How to scale a Texture in an Atlas without seeing neighbouring Textures
Remap the coordinates.
We benefit from the fact we know it is a Texture Atlas, with regular row and column sizes. Take the UV coordinates, check the local X and Y coordinates and remap it to the current Texture by adding it to the Start X and Y of the target Texture.

Result (With a scale of 6)

Unfortunately the individual textures are not seamless as it seems, so you will see Seams every time the UV snap back to the start coordinates.
Small helping addition if you are using V 2.92+. Create two custom properties in the object properties.

Instead of the Current Column and Row nodes, use Attribute nodes, set the type to Object, and type in the name of your attributes.

Now you can directly edit the Texture in the 3D Viewport :

Also, if you want to have a random Texture with each pot when you duplicate it :
Use the Random output from the Object Info node to randomize Atlas Position.

Result :